This invention has relation to hydraulic circuitry for a backhoe excavator. The same pair of hydraulic pumps is used to power a first set of functions such as one side of a crawler track drive, and also a second or other set of functions such as boom, bucket, stick or auxiliary drum drives.
It is desirable to perform the no-load movement of hydraulic piston-cylinder motors at relatively high speed to achieve minimum operating times for movement of boom, bucket and stick, for example. If this is achieved by use of a constant volume source of hydraulic fluid, when resistance to movement is encountered in a particular motor, as when coming under load, the pressure inside the motor will rapidly exceed the safe overload pressure and a large volume of fluid will pass out of the overload relief valve back to the fluid tank or reservoir. This is a waste of power, and occasions excessive heating of the fluid which must be compensated for by incorporation of an overly large hydraulic fluid cooling system.
If no-load movement of the hydraulic piston-cylinder motors is achieved at relatively high speed by use of large, constant horsepower control, pressure compensated variable displacement pumps as sometimes employed in prior art structures common in Europe, the flow volume will begin decreasing when something like one-half of the maximum pressure develops in the system under load, and this slow down of movement of the piston-cylinder motors will continue until the saft overload pressure is exceeded, at which point about 40% of the pumps maximum volume will pass over the relief valve and back to the fluid tank or reservoir, thus constituting a waste of power and excessive heating of the fluid.
One way to overcome these problems is by the use of two sources of fluid and an unloading valve which disconnects one of the sources from the motor and bypasses it to the return line under low pressure when the pressure in the motor lines exceeds a predetermined pressure less than the relief valve pressure.
Such high speed to low speed hydraulic systems are well known. See the patent to Vickers, U.S. Pat. No. 1,982,711, issued Dec. 4, 1934, in which pumps 1 and 2 furnish hydraulic fluid to move a power cylinder 22 until such time as a predetermined high pressure is built up in cylinder 22. At that time, an unloading valve unloads pump 2 so that Pump 1 can continue to activate the power cylinder, but at slow speed. When the pressure in cylinder 22 is reduced, as when the direction of hydraulic fluid flow into the cylinder is reversed, the unloading valve allows the hydraulic fluid from pump 2 to once again be introduced into the system for high speed operation.
The use of an unloading valve in a hydraulic circuit incorporating two hydraulic pumps to unload one of the pumps when a considerable resistance is encountered is also shown in the patent to LaRou, U.S. Pat. No. 3,156,098, issued Nov. 10, 1964. This patent is related to backhoes and diggers.
Use of a hydraulic circuit for an excavator with means to propel the excavator and associated means to power the implement is shown in the patent to Metailler, U.S. Pat. No. 3,172,552, issued Mar. 9, 1965. See FIG. 6.
Use of a low volume pump and a high volume pump to perform various functions in a backhoe vehicle is shown in the patent to Arnold, U.S. Pat. No. 3,257,013, issued June 21, 1966. A manually operable selector valve 86 can be activated to direct the output of one of the pumps to add to the output of the other pump when high speed operation is advantageous. The difficulty here is that when high resistance and therefore high pressure is encountered with the controls in that position, the overload valves will be activated, thus dumping the hydraulic fluid into the return tank, causing power waste and undue overheating of the hydraulic fluid. See FIG. 6 of Arnold.
Use of a plurality of fluid motors to operate backhoe functions at high speeds under light loads and at low speeds but higher fluid pressures under heavy loads is shown in the patent to Stacey, U.S. Pat. No. 3,146,593, issued Sept. 1, 1964.
A patent which shows two pumps operating independent track drives on a backhoe excavator is the patent to Morrison et al., U.S. Pat. No. 3,466,770, issued Sept. 16, 1969. The valving here is through a single manifold 196, however. See FIG. 7.
In the past, in order to prevent duplication of hydraulic pump circuits and controls, it has been customary to forego the ability to move a hydraulic backhoe on its crawler assembly while performing full force operations with the bucket, stick and boom.